Monomeric guanine nucleotide-binding proteins of the ras superfamily of 18-30 kDa function in a variety of cellular processes including signaling, growth, immunity, and protein transport (Barbacid, H. Annu. Rev. Biochem. 56: 779-828 (1987); Bourne, H. R. Cell 53:669-671 (1988); Bourne, et al. Nature (London) 349:117-127 (1991); Gabig, et al. J. Biol. Chem. 262: 1685-1690 (1987); Goud, et al. Nature (London) 345:553-556 (1990); Hall, A. Science 249:635-640 (1990); Knaus, et al. Science 254: 1512-1515 (1991)). ADP-ribosylation factors (ARFs) constitute one family of the ras superfamily.
ARF was initially identified as a factor required for cholera toxdin-catalyzed ADP-ribosylation of G.sub.sa, the stimulatory guanine nucleotide-binding (G) protein of the adenylylcyclase system (Kahn, et al. J. Biol. Chem. 259: 6228-6234 (1984); Serventi, et al. In: Current Topics in Microbiology and Immunology 175, (Aktories, K. ed) pp. 43-67, Springer-Verlag, Berlin Heidelberg (1992) . In the presence of GTP or a nonhydrolyzable GTP analogue, ARF serves as an allosteric activator of cholera toxin ADP-ribosyltransferase (Noda, et al. Biochim. Biophys. Acta 1034: 195-199 (1990); Tsai, et al. J. Biol. Chem. 263: 1768-1772 (1988); Tsai, et al. Proc. Natl. Acad. Sci. (USA) 84: 5139-5142 (1987)). It stimulates the toxin-catalyzed ADP-ribosylation of proteins unrelated to G.sub.sa and simple guanidino compounds such as arginine and agmatine as well as auto-ADP-ribosylation of the toxin A1 protein (Noda, et al. Biochim. Biophys. Acta 1034: 195-199 (1990); Tsai, et al. J. Biol. Chem. 263:1768-1772 (1988); Tsai, et al. Proc. Natl. Acad. Sci. (USA) 84:5139-5142 (1987)).
ARFs are evolutionarily well conserved and present in all eukaryotes from Giardia to mammals (Kahn, et al. J. Biol. Chem. 263:8282-8287 (1988); Murtagh, et al. J. Biol. Chem. 267:9654-9662 (1992); Tsai, et al. J. Biol. Chem. 266: 8213-8219 (1991); Tsuchiya, et al. Biochemistry 28: 9668-9673 (1989); Tsuchiya, et al. J. Biol. Chem. 266: 2772-2777 (1991)). Immunologically, they have been localized to the Golgi apparatus of several types of cells (Stearns et al. Proc. Natl. Acad. Sci. (USA) 87:1238-1242 (1990)). ARFs are required for association of nonclathrin coat proteins with intracellular transport vesicles (Serafini, et al. Cell 67: 239-253 (1991)) and also appear to be critical during an early step in endocytosis as well as in nuclear vesicle fusion (Boman, et al. Nature (London) 358: 512-514 (1992); Lenhard, et al. J. Biol. Chem. 267:13047-13052 (1992)). GTP binding and hydrolysis may be involved in binding of ARF to membranes, and the nonhydrolyzable GTP analogue GTP.sub..gamma. S, but not GTP or GDP, promotes the association of cytosolic ARF with Golgi (Regazzi, et al. Biochem. J. 275:639-644 (1991)) 1991) or phospholipid membranes (Kahn, et al. J. Biol. Chem. 266:15595-15597 (1991); Walker, et al. J. Biol. Chem. 267: 3230-3235 (1992)).
By molecular cloning from cDNA and genomic libraries, and PCR amplification of RNA transcripts, six mammalian ARFs, two yeast ARFs, and two Giardia ARFs have been identified (Bobak, et al. Proc. Natl. Acad. Sci. (USA) 86: 6101-6105 (1989); Monaco, et al. Proc. Natl. Acad. Sci. (USA) 87: 2206-2210 (1990); Murtagh, et al. J. Biol. Chem. 267:9654-9662 (1992); Price, et al. Proc. Natl. Acad. Sci. (USA) 85: 5488-5491 (1988); Sewell, et al. Proc. Natl. Acad. Sci. (USA) 85: 4620-4624 (1988); Stearns, et al. Mol. Cell. Biol. 10: 6690-6699 (1990); Tsuchiya, et al. J. Biol. Chem. 266:2772-2777 (1991)). Mammalian ARFs fall into three classes based on deduced amino acid sequences, gene structure, phylogenetic analysis, and size (Lee, et al. J. Biol. Chem. 267:9028-9034 (1992); Tsuchiya, et al. J. Biol. Chem. 266: 2772-2777 (1991)). Class I ARFs are ARFs 1-3; class II includes ARFs 4 and 5; and class III has ARFs 6. Some lipids and/or detergents, e.g., SDS, cardiolipin, dimyristoylphosphatidylcholine (DMPC)/cholate, enhance ARF activities (Bobak, et al. Biochemistry 29:855-861 (1990); Noda, et al. Biochim. Biophys. Acta 1034: 195-199 (1990); Tsai, et al. J. Biol. Chem. 263:1768-1772 (1988)). Members of the ARF multigene family, when expressed as recombinant proteins in E. coli, display different phospholipid and detergent requirements (Price, et al. J. Biol. Chem. 267: 17766-17772 (1992)). Following synthesis in E. coli all of these ARFs had enhanced cholera toxin ADP-ribosyltransferase activity in the presence of GTP' (Kahn, et al. J. Biol. Chem. 266: 2606-2614 (1991); Price, et al. J. Biol. Chem. 267: 17766-17772 (1992); Weiss, et al. J. Biol. Chem. 264: 21066-21072 (1989)).
In general, differences in the various ARF sequences are concentrated in the amino-terminal regions and the carboxyl portions of the proteins. Only three of 17 amino acids includin Met.sup.1 and Gly.sup.2, in the amino termini are identical among ARFs, and four amino acids in this region of ARFs 1-5 are missing in ARF 6 (Tsuchiya, et al. J. Biol. Chem. 266: 2772-2777 (1991)). It was recently reported (Kahn, et al. J. Biol. Chem. 267:13039-13046 (1992)) that the amino-terminal regions of ARF proteins form an .alpha.-helix, and that this domain is required for membrane targeting, interaction with lipid, and ARF activity.
Schliefer et al., (J. Biol. Chem. 257: 20-23 (1991)) described a protein distinctly larger than ARF that possessed ARF-like activity. At the time of those studies however, it had not been demonstrated that ARF requires GTP for activity, so functional characterization of the protein did not include assessment of that property.